One important thing to note is that if the sequel takes place a few years after the first film, would Grace's Avatar be able to survive in a vegetative state for that long? Is there a way to keep it healthy for an extended period of time?

I'd say yes. Even after a few years, it should still survive since it seems that when not connected, they are in what seems like a state of deep hibernation and minimal process to the point that even basic functions like breathing are very low, as with no neural activity other than maintaining control of autonomic functions, there is little energy requirement, especially if the hibernation extends to a cellular level. She would certainly be weak at first after a few years, but could easily recover with time. Of course,putting it in an amnio talk would be no problem either, especially with Max and various other scientists still around